The present invention broadly relates to smoke detectors and, more specifically, pertains to a new and improved construction of a photoelectric smoke detector.
Generally speaking, the photoelectric smoke detector of the present invention relates to a photoelectric smoke detector having a source of radiation driven intermittently by a control circuit and a radiation receiver which is connected to an evaluation circuit which is able to transmit a smoke alarm signal when the radiation receiver receives radition that has been affected by smoke particles in synchronous operation with the radiation source.
In other words, the photoelectric smoke detector of the present invention comprises a radiation source, a control circuit for intermittently driving the radiation source and a radiation receiver. The radiation receiver is connected to an evaluation circuit capable of outputting an alternating smoke alarm signal having at least one phase when the radiation receiver receives radiation influenced by smoke particles in synchronization with operation of the radiation source.
That is, the photoelectric smoke detector of the present invention is of the type comprising a source of radiation for emitting radiation, a control circuit for generating an alternating signal exhibiting a phase and for intermittently driving the source of radiation in synchronism with the phase of the altnerating signal, a radiation receiver for receiving a portion of the radiation influenced by smoke particles and for generating an output signal indicative of the received portion and exhibiting a polarity as well as an evaluation circuit connected to the radiation receiver and to the control circuit for generating a smoke alarm signal when the radiation receiver receives the portion of the radiation influenced by smoke particles in synchronism with the radiation intermittently emitted by the source of radiation.
The present invention also relates to a fire alarm device for detecting smoke particles generated by a fire and comprising a photoelectric smoke detector of the inventive type.
The smoke detector can, for example, be structured as a scattered radiation detector in which the radiation scattered from smoke particles is evaluated, or as a radiation extinction detector which exploits the diminution of radiation or its absorption by smoke particles, or as a photo-acoustic smoke detector in which the smoke particles emit acoustic pulses upon absorbing radiation pulses and an acoustical-electrical converter transforms them into electrical pulses, such as is described in European Patent Application EP No. 14,251. The smoke detector can also serve as a smoke sensor in which the value of the emitted smoke alarm signal is an indication of the smoke intensity. It can also serve as a smoke alarm which gives warning of a prescribed intensity of smoke.
In such smoke detectors, such as for example the scattered radiation smoke detector described in PCT Application WO No. 80/01326, which are preferably employed as fire alarms, electromagnetic radiation is radiated into a measuring volume, for instance by means of a light-emitting diode (LED) and the radiation scattered from smoke particles in the measuring volume is received by a scattered radiation receiver disposed outside of the radiating zone of the radiation source. An evaluation circuit outputs a smoke alarm signal when the level of scattered radiation exceeds a prescribed threshold. Electromagnetic radiation is to be understood as including visible light, infrared radiation or ultraviolet radiation.
A key problem in the art is to assure that a smoke alarm signal only be initiated by radiation scattered from smoke particles and not by interference radiation penetrating into the measuring volume which could equally well be picked up by the radiation receiver and falsely indicate the presence of radiation-scattering smoke particles. This is particularly important in smoke detectors in which only a limited intensity of radiation is available in the measuring volume, for instance in smoke detectors in which the radiation is conducted to and fed back from the measuring volume by means of radiation conducting elements or fiber optics, as for example is described in German Patent Application No. 3,037,636.
In order to prevent interference radiation from giving false smoke alarm signals it is known, for example from European Patent No. 11,205 or European Patent No. 14,799, to operate the radiation source in very short pulses and to connect the radiation receiver to a coincidence circuit which only generates a smoke alarm signal when scattered radiation is received during the short radiation pulse periods but not during response to the occurrence of the interference radiation pulses in the intermediate intervals. The rare case in which an interference pulse is received during the short period of radiation pulse can be eliminated by a repetition circuit which only re-transmits a smoke signal when a plurality of coincidences occur in sequence.
As far as sufficiently intensive radiation pulses are available, fire alarms insensitive to interference can be obtained through the use of such evaluation circuits. Many radiation sources, however, can only deliver a limited maximum intensity without damage or premature aging and furthermore there is radiation damping in fiber optics transmission so that it is advantageous or necessary to provide longer active intervals of the radiation source in order to obtain sufficient scattered radiation capacity. The evaluation circuits mentioned then do not operate with sufficient intensity to interference, since on the one hand, the occurrence of interference pulses during the active intervals is much more probable, and since on the other hand, the signal-to-noise ratio in the radiation receiver can become so small that some noise pulses attain the signal level and give rise to a false alarm signal. Particularly weak concentrations of smoke, in which the signal lies below the noise level, could not be detected at all in this manner, i.e., the sensitivity of fire alarms having such evaluation circuits is limited.